1. Field of the Invention
The invention relates to a ball screw device and an electric power steering system including the ball screw device.
2. Description of Related Art
In related art, there is a so-called rack assist type electric power steering system including a hollow shaft in which a rack shaft is inserted and which is rotated by motor drive, the electric power steering system applying an assist force to a steering system by converting the rotation of the hollow shaft to the axial movement of the rack shaft by a ball screw device (for example, US 2011/0296939 A1).
In general, such a ball screw device is formed by disposing a plurality of balls in a spiral ball raceway which is formed by a thread groove formed in the outer periphery of the rack shaft and a thread groove formed in the inner periphery of a ball screw nut, the thread groove formed in the outer periphery of the rack shaft facing the thread groove formed in the inner periphery of the ball screw nut. Each ball in the ball raceway is sandwiched between the rack shaft and the ball screw nut, and rolls in the ball raceway under load (a frictional force) from the ball screw nut and the rack shaft when the ball screw nut rotates relatively with respect to the rack shaft. Further, the ball screw device has a circulation path that connects two points set in the ball raceway. Each ball that rolls in the ball raceway passes through the circulation path, thereby moving between the two points, that is, returning from the downstream side to the upstream side. When the ball newly enters the circulation path from the ball raceway, each ball in the circulation path is pushed by the other ball adjacent to and posterior to the ball in a circulation direction (a moving direction) of the balls. Thus, the balls move in the circulation path. In the ball screw device, the balls infinitely circulate in a movement pathway which is formed by the ball raceway and the circulation path, thereby converting the rotation of the ball screw nut to the axial movement of the rack shaft.
As for the ball screw device, as described in US 2011/0296939 A1, a so-called deflector type ball screw device is known in which the circulation path is configured by fitting a deflector having a function of taking up balls from a ball raceway and a function of discharging the balls to the ball raceway, in a fitting hole that radially extends through the ball screw nut. In the deflector type ball screw device, a difference in level may be caused in a joint portion between the ball raceway and the circulation path due to a dimensional accuracy of the deflector, or the like. Therefore, for example, in a ball screw device described in Japanese Patent Application Publication No. 11-270648 (JP 11-270648 A), a difference in level is eliminated by performing level difference processing such as polishing on a joint portion between a deflector and a ball screw nut after the deflector is fitted.
As a structure for fixing the ball screw nut to the hollow shaft, for example, as described in US 2011/0296939 A1, a structure is known in which a hollow cylindrical male thread portion is formed in the ball screw nut, and the male thread portion is engaged with a female thread portion formed in the hollow shaft so as to fasten the ball screw nut to the hollow shaft. In order to restrict the rotation of the hollow shaft when fastening the ball screw nut, it is conceivable that a cut portion is formed in a seating surface (an axial end face that axially faces the ball screw nut) of the hollow shaft and a jig for fixing is engaged with the cut portion.
However, if the cut portion is formed in the seating surface, when the ball screw nut is pressed against the seating surface by an axial force that is generated by the engagement of the male thread portion and the female thread portion, a site in the ball screw nut, which faces the cut portion, is inserted into the cut portion so as to bulge out into the cut portion, whereby the ball screw nut is easily deformed. As a result, there is a problem that a distortion occurs in the thread groove of the ball screw nut, whereby smooth circulation of the balls in the movement pathway is hindered and for example, abnormal noise is caused.